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Home Interviews How do you read a file line by line using a generator-like approach (e.g., yield return)? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 11 May 2025
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You can create a generator-like method using yield return to read a file line by line. This is memory-efficient, especially for large files, because it reads one line at a time and doesn’t load the whole file into memory.

Example: Generator-style file reader

using System;
using System.Collections.Generic;
using System.IO;

class Program
{
    static void Main()
    {
        string path = @"C:\example.txt";

        foreach (var line in ReadLines(path))
        {
            Console.WriteLine(line);
        }
    }

    static IEnumerable<string> ReadLines(string filePath)
    {
        using (var reader = new StreamReader(filePath))
        {
            string? line;
            while ((line = reader.ReadLine()) != null)
            {
                yield return line;
            }
        }
    }
}

Why use yield return here?

  1. Defers execution: lines are read only when needed.
  2. Saves memory: no need to store all lines in a list or array.
  3. Makes the code composable with LINQ or foreach.

You can combine this with LINQ filters like:

var longLines = ReadLines("file.txt").Where(l => l.Length > 80);
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